In the recent times, there is renewed focus on Hindi and how the language can be a unifier for the entire nation. However, it is still a language that is unknown to a vast number of Indians. For example, Indians hailing from the South are unable to converse freely in Hindi. English, on the other hand, is considered as an official language used all over the country. All competitive exams have questions in English, though it may not be so for Hindi.
This clearly states that English is the only language which should be used for SSB interviews where Indians from the entire length and breadth of the country may be present.
Here are some reasons why English is your best bet at SSB interviews:
The assessing officers will address you in English. Questions will be directed to you in the same language. During the PPDT round, or during GD, you will be expected to speak in English. Yes, you may use Hindi if you get stuck with a sentence but you have to fall back on English to complete your point.
Other than the board of officers assessing your performance, you have to talk to other candidates. They may not be free with Hindi. Candidates from the North East, for example, may be ill-equipped to talk in Hindi. To engage them in discussions, you need English. Similarly, they will be using English instead of their native tongue for your benefit. English becomes that platform for all.
During your stint as a defence service officer, you may be posted at any corner of the country. Here too, English will be the language you can use to communicate. Before you crack SSB, assessing officers will evaluate whether you know English well enough for this purpose as well.
As a team member or leader of a group in the defence forces, you will have to follow or give orders. Unless they are in a language that is known to all present, the orders may be misinterpreted or misunderstood, which is worse. You need to be well-conversant in English in order to ensure that language does not become a barrier in your line of duty.
Instead of trying to work your way around the problem of knowing and speaking in English, try to embrace the language as a necessity for a career in the defence forces.